Lithium soap grease has conventionally been used widely because of its relatively stable heat resistance and water resistance and also because most lithium soaps are easily dispersible in lubricant bases and inexpensive. However, when used in a high temperature environment of 130.degree. C. or higher, lithium soap grease suffers deterioration in characteristics by heat, such as destruction of micelles, reduction in adhesiveness, and softening, thereby abruptly reducing its lubricating action.
Heat-resistant grease compositions compensating for the above disadvantage, such as soapless grease having an extremely high dropping point (i.e., excellent thermal stability) and complex soap grease of various types, have been developed, but they are disadvantageous in that the thickening agent used hardens or extremely softens in long-term use. Besides, they are very expensive and have therefore found limited use.
In recent years, the working environment of grease is getting severer with the size reduction of machinery and increase in speed of working parts. For example, when used for lubrication in places close to an engine heat source, as in bearings for electrical equipment in automobiles, the grease used for lubricating the bearings is exposed to a particularly high temperature for a long time. Further, constant velocity joints (CVJ), which are often used in FWD (front wheel drive) cars or 4WD (four wheel drive) cars, are used under severe conditions due to high output power and high speed of the cars and weight reduction of CVJ themselves. Such being the case, the grease to be used must have sufficient heat resistance and durability.
In appliances, small-sized bearings used in small-sized motors of air conditioners, fans, etc. and various audio parts are required to have low vibration and low noise. Hence, grease having satisfactory bearing acoustic characteristics (making no noise) as well as excellent lubricating properties are demanded.
On the other hand, with the industrial advancement, industrial machinery has been achieving high performance, getting more compact, and working under severer conditions, thus needing lubricating grease withstanding such use. The state-of-the-art load resistant grease contains a large quantity of extreme pressure additives for fulfilling various requirements under high load. However, extreme pressure additives are liable to cause environmental pollution or corrosion depending on choice. For example, of the conventional additives providing satisfactory extreme pressure properties, the use of those containing cadmium or antimony is now forbidden due to their toxicity. Further, chlorine-containing additives give corrosive influences to iron, steel, etc. particularly at 100.degree. C. or higher.
The use of boric acid salts as grease additives for improving load resistance has been proposed in JP-A-51-33263 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), U.S. Pat. No. 3,997,454, JP-A-53-115704, JP-A-53-115705, U.S. Pat. No. 4,100,080, and JP-A-59-109595. In particular, the grease using a borate dispersion as disclosed in U.S. Pat. No. 3,997,454 is excellent. However, the borate dispersion disclosed is a dispersion of a borate in a mixture consisting of an alkali metal sulfonate or an alkaline earth metal sulfonate and an alkenylsuccinimide, and it does not per se have an anti-oxidant action.